1. Field of the Invention
The present invention relates to a method of and apparatus for optically measuring a displacement which are utilized by way of example for the displacement measurement of a machine requiring accurate positioning such as a semiconductor manufacturing plant or a coordinate measuring instrument.
2. Description of the Prior Art
A method of optically measuring the displacement of an object to-be-measured is, for example, an interferometric method disclosed in the specification of U.S. Pat. No. 4,298,283. This method is such that light is projected on a reference plane and a plane to-be-measured, that the phases of resulting reflected lights are cyclically and forcibly changed by a phase shifter, and that the magnitude of the displacement of the object to-be-measured is evaluated from the magnitude of phase shift at the moment at which the phase change generated by the displacement of the object has been compensated by the phase shifter.
This method, however, cannot be used for a positioning displacement sensor because the measurement range thereof is as narrow as approximately a half of the wavelength of a light source employed. Moreover, even when the use is restricted to the measurements of minute displacements, the limited measurement range makes it necessary to adjust the phases of reflected lights with a Babinet-Soleil compensator before the measurement and to start the measurement from the middle of the measurement range. This initialization requires a long time.